In general the player assumes that it won't receive out of order video.

In general the player assumes that it won't receive out of order video.

This clearly can happen with separate L/R sources.  A pass in L might
emit two frames which means the arrivals can't possibly be in order.

This commit fixes this by introducing a Shuffler which all alternate-3D
sources send their video to.  The Shuffler re-orders things before they
arrive at the player.

It also fixes the code which inserts video frames before one that arrives
after a gap.  This didn't cope with 3D right before.

The audio code solves a similar (perhaps the same?) problem with the
AudioMerger; perhaps we should have a similar thing for video and make
the player emit complete 3D frames.

Should help with #976.